Optically controlled switches

ABSTRACT

An optically controlled switch includes first and second electrodes, a channel extending between the electrodes, and a light source positioned to illuminate the channel. The light source produces a wavelength capable of changing the material&#39;s conductivity. The channel includes a photosensitive organic material and is configured to operate as a light controlled switch.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to photosensitive electrical devices.

[0003] 2. Discussion of the Related Art

[0004] Many complex systems use electrical control circuits to operateother devices. Some such electrical control circuits use photosensitivematerials to control the currents or voltages therein. Thephotosensitive materials include semiconductors such as gallium arsenide(GaAs).

[0005] In a semiconductor, light of an appropriate wavelength opticallyexcites mobile carriers. The optical generation of mobile carriersreduces the resistance of a channel made of the semiconductor. Theoptically induced change in channel resistance has been used as atrigger for such electrical control circuits.

SUMMARY OF THE INVENTION

[0006] When a conventional semiconductor is not illuminated, thematerial still has a significant conductivity. Thus, a channel made froma conventional semiconductor typically supports a significant leakagecurrent when not illuminated. Due to the high leakage current, aconventional semiconductor channel does not function like opticallycontrolled switch.

[0007] Various embodiments according to principles of the inventionprovide a photosensitive switch. The photosensitive switch has aconducting state in which the switch supports a substantial current andan insulating state in which the switch supports, at most, a low leakagecurrent. The photosensitive switch goes rapidly from the insulatingstate to conducting state when illuminated by light of an appropriatewavelength. The photosensitive switch is advantageous as a regulator fora high voltage source, because the switch passes, at most, a low leakagecurrent when not illuminated.

[0008] One optically controlled switch according to principles of theinvention includes first and second electrodes, a channel extendingbetween the electrodes, and a light source. The channel includes aphotosensitive organic material. The light source is capable ofilluminating the entire length of the channel and of changing thechannel from an insulating state to a conducting state.

BRIEF DESCRIPTION OF THE FIGURES

[0009]FIG. 1 is a cross-sectional view of an optically controlledswitch;

[0010]FIG. 2 shows a control circuit based on the optically controlledswitch of FIG. 1;

[0011]FIG. 3 is a flow chart for a method of operating theoptically-based control circuit of FIG. 2; and FIG. 4 is an oblique viewof a micro-electromechanical (MEM) device that uses the optically-basedcontrol circuit of FIG. 2.

[0012] In the Figures, like reference numbers refer to functionallyequivalent elements or features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0013]FIG. 1 shows an optically controlled switch 10. The switch 10includes a photosensitive switch 12 and a light source 14. Thephotosensitive switch 12 is based on a planar structure. The planarstructure includes an insulating substrate 16, two electrodes 18, 20located on the substrate 16, and a photosensitive layer 22 that overlaysboth electrodes 18, 20 and the substrate 16. The light source 14produces light with a wavelength that is adapted to change theresistivity of the material in the photosensitive layer 22.

[0014] In the planar topology, the thickness of photosensitive layer 22is less than the length of channel region 26. Also, light source 14transmits light in a direction transverse to the conduction direction,L, in channel region 26. Thus, the light is able to penetrate the entirelength of the channel region 26 even if the channel region 26 is long.

[0015] For switch-like behavior, the ratio of the resistance of channelregion 26 when illuminated, i.e., bright state, to the resistance of thechannel region 26 when not illuminated, i.e., dark state, must be atleast 10⁴, preferably is at least 10⁶, and more preferably is 10⁸ ormore. To obtain such a high ratio of resistances, the entire length ofthe channel region 26 must illuminated by light source 14 in the brightstate. If a small transverse section along the channel region 26 remainsinsulating in the bright state, the resistance of that portion willdominate the entire channel resistance, because the resistivity of thechannel material is orders of magnitude larger in the insulating statethan in the conducting state. Thus, if a small section along the lengthof the channel region 26 remains non-illuminated, the ratio of thedark-state resistance to the bright-state resistance will not have thelarger values characteristic of switch behavior.

[0016] This should be contrasted with a stacked topology common to solarcells (not shown). In a stacked topology, incident light propagatesalong the direction of current flow in the channel region. The length ofthe channel region must be short if light is to penetrate the entirelength of the channel region.

[0017] In the planar topology, channel region 26 may be as long asdesired without interfering with the ability of light source 14 toilluminate the entire channel region 26. In contrast with the stackedtopology, the planar topology enables the channel length to be longenough to provide a high channel breakdown voltage without interferingwith the need for the whole channel region 26 to be conducting in thebright state. Exemplary breakdown voltages for channel region 26 are atleast 50 volts, preferably at least 100 volts and more preferably atleast 300 volts.

[0018] The planar topology also allows channel region 26 to have adark-state electrical resistance characteristic of switch behavior,i.e., due to the long channel length. Exemplary channel regions 26 havedark-state resistances of at least 10⁷ ohms, preferably at least 10⁸ohms, and more preferably 10⁹ ohms or more. These large resistancesinsure that photosensitive switch 12 has a very low leakage current inthe dark state.

[0019] In FIG. 1, the electrodes 18, 20 are made of gold (Au), aluminum(Al), indium-tin-oxide, titanium nitride (TiN), heavily doped silicon,or other conductors. In preferred embodiments, both electrodes 18, 20are made from the same conductor so that illumination does notphotovoltaically produce a voltage across channel region 26.

[0020] The material of photosensitive layer 22 has a resistivity thatresponds to light in a preselected wavelength range. When notilluminated, the photosensitive layer 22 is a good insulator, and whenilluminated, the photosensitive layer 22 is a fairly good conductor. Forchannel region 26, the ratio of the resistance in the dark state to theresistance in the light state is significantly higher than for inorganicsemiconductors.

[0021] The photosensitive layer 22 includes an organic matrix that isdoped with an appropriate electron donor or acceptor to produce amaterial that conducts when suitably illuminated.

[0022] Exemplary organic materials for photosensitive layer 22, includeconjugated organic oligomers and polymers such as derivatives ofoligomers and polymers containing aromatic units such asphenylenevinylenes, fluorenes, thiophenes, and pyrroles. Exemplaryoligomers and polymers of phenylenevinylenes have substitutions ofalkoxyl or cyano groups off the main chains. Some matrices includecopolymers and blends of one or more of the above-described conjugatedorganic oligomers and polymers.

[0023] Preferred organic materials are fully conjugated oligomers and/orpolymers that are molecularly aligned to increase the conductivitybetween electrodes 18, 20 when suitably illuminated. The preferredalignments increase inter-molecular overlaps to provide higher chargemobilities when suitably illuminated, e.g., mobilities of about 10⁻⁶cm²/volt-second or more. The matrix molecules may be aligned bystretching a matrix film prior to deposition, quenching the matrix to aliquid crystal state from a liquid state, or depositing the matrix on analignment layer.

[0024] Exemplary dopants for organic matrices include organic oligomersand polymers, inorganic nanocrystals, and organo-metallic complexes. Thedopants are either miscible in the organic matrix or chemically bound tothe matrix molecules. Upon illumination, the dopants function as eitherelectron donors or electron acceptors for the matrix, which wouldotherwise be an insulator.

[0025] The systems of dopants and matrix molecules belong to one of twoclasses. In the first class, the dopants are acceptors of photo-excitedelectrons from the organic matrix or donors of photo-excited holes tothe matrix. In the second class, the dopants are photo-excitable donorsof electrons to the organic matrix or acceptors of photo-excited holesfrom the matrix. Photo-excitations can result from the absorption oflight by either the matrix molecules or dopants. Each class involves aparticular alignment between highest occupied molecular orbitals (HOMOs)and lowest unoccupied molecular orbitals (LUMOs) of the dopants andmatrix molecules.

[0026] In the first class, the HOMO of the matrix molecules has a higherenergy than the HOMO of the dopants, and the LUMO of the matrixmolecules also has a higher energy than the LUMO of the dopants. Forthis alignment of energy levels, dopants have higher electron affinitiesand higher ionization potentials than matrix molecules. Exemplary ofthis class are systems in which the matrix includespoly(dialkoxyphenylenevinylene)s and the dopants are selected from C₆₀,metal-phthalocyanines, thia-pyrylium, squarylium, azo-compounds,perylene, anthanthrone, and nanocrystalline CdSe.

[0027] In the second class, the HOMO of the matrix molecules has a lowerenergy than the HOMO of the dopants, and the LUMO of the matrixmolecules also has a lower energy than the LUMO of the dopants. For thisorbital alignment, the dopants have lower electron affinities and lowerionization potentials than the matrix molecules. Exemplary of the classare systems where the matrix includespoly(α,α′-dicyanophenylenevinylene)s and the dopants arepoly(dialkoxyphenylenevinylene)s.

[0028] In photosensitive layer 22, dopant concentrations are fixed toproduce desired conductivities when suitably illuminated by light source14. Preferred conductivities result from between about 10¹⁹ and about10²¹ mobile charge carriers per centimeter cubed when suitablyilluminated. To achieve such charge carrier concentrations, organicmaterials include significant volume fractions of dopants. The volumefraction occupied by dopants is typically greater than 0.1 percent,preferably at least 1.0 percent, and often 10 percent or more.

[0029] Light source 14 excites electrons either from dopant sites to thematrix or from the matrix to dopant sites to convert photosensitivelayer 22 from an insulating state to a conducting state. Thus, theconductivity of photosensitive layer 22 depends on both the dopantdensity and the illumination intensity from the light source 14. Thedependencies of the conductivity on the dopant density and theillumination intensity are often approximately linear.

[0030] The conductivity of channel region 26 varies linearly with boththe channel width and the inverse of the channel length. A preselecteddark-state resistance fixes the ratio of the width to length of thechannel region 26. The dark-state resistance determines the leakagecurrent through the photosensitive switch 12. A desired minimumbreakdown voltage determines the minimum length for the channel region26 of the photosensitive switch 12.

[0031] A person of skill in the art could determine suitable channeldimensions and dopant fractions based on preselected values of thedark-state and light-state channel resistances, the intensity of lightsource 14, and the channel breakdown voltage.

[0032]FIG. 2 shows a control circuit 34 based on optically controlledswitch 10 of FIG. 1. The control circuit 34 includes a direct current(DC) voltage source 36 and a voltage divider 38. In the voltage divider38, the optically controlled switch 10 and a fixed resistor 40 connectin series. The fixed resistor 40 is a voltage source for a load element42, e.g., a capacitor or inductor. The resistance of the opticallycontrolled switch 10 controls the current through the fixed resistor 40and thus, the voltage drop applied across the load element 42.

[0033] The optically controlled switch 10 includes light source 14 andphotosensitive switch 12 of FIG. 1. Exemplary light sources 14 includelight emitting diodes (LED) and diode lasers. The light source 14 mayinclude an optical waveguide, e.g., an optical fiber, that deliverslight from a remote source to the photosensitive switch 12. A voltage,V, used to modulate the light source 14 controls the resistance ofphotosensitive switch 12.

[0034]FIG. 3 is a flow chart for a method 44 of controlling a circuitvia an optically controlled variable switch, e.g., switch 12 of FIG. 3.The method 44 includes applying an external voltage across aphotosensitive switch located in the circuit (step 46). The method 44also includes modulating the intensity of a light source, e.g. lightsource 14 of FIG. 2, that illuminates the photosensitive organicresistor while the external voltage is applied across the photosensitiveorganic switch (step 48). The modulated light intensity changes theresistance of the photosensitive switch and thus, the current that theexternal voltage produces in the circuit. The changed current changesthe voltage drop across a load element, e.g., load element 42 in FIG. 2.

[0035] The induced change in the voltage drop across the photosensitiveswitch is greater than any photovoltaic voltage induced across thephotosensitive switch. Preferably, the change in the voltage drop is atleast ten times any produced photovoltaic voltage.

[0036] Referring again to FIG. 2, exemplary control circuit 34 functionsas a digitally modulated (DM) voltage source for load element 42. In theDM voltage source, light source 14 functions as an optical modulatorthat produces a repeating sequence of bright and dark periods, e.g., ONand OFF periods of a diode laser or LED. The relative lengths of thebright and dark periods are varied to apply different average voltagesacross fixed resistor 40 and load element 42.

[0037]FIG. 4 shows a micro-electromechanical (MEM) device 50 controlledby control circuit 34 of FIG. 3. The MEM device 50 includes a flexiblestalk 52 and a top piece 54. The stalk 52 connects the top piece 54 tosubstrate 16. The top piece 54 includes a first plate 56 of a capacitorand a reflector 58. A second plate 60 of the capacitor is located on thesubstrate 16. The capacitor is load element 42 of the control circuit 34shown in FIG. 3. The control circuit 34 determines the charge state ofthe capacitor thereby controlling the orientation of the reflector 58 onthe MEM device 50.

[0038] The control circuit 34 functions as a DM voltage source forcharging the capacitor that controls the orientation of MEM device 50.In the DM voltage source, light source 14 shines a light beam with amodulated intensity on photosensitive resistor 12. The light intensityis modulated at a frequency that is higher than the time constant formechanical resonance in the MEM device 50, e.g., at least 5-10 times themechanical resonance frequency. At such high frequencies, the averagecharge on plates 56, 60 determines the mechanical reaction of MEM device50 to the driving voltage. The average charge on the plates 56, 60depends on the relative lengths of the bright and dark portions of theillumination cycle.

[0039] Digital modulation of light source 14 requires a high frequencyvoltage source, V. The voltage source, V, can be a digital source, butthe voltage source, V, typically has a maximum amplitude that is muchsmaller than that of the voltage modulating the charging and dischargingof the capacitor of MEM device 50. The voltage applied to capacitor istypically in the range of 0 volts-1000 volts and is preferably in therange of about 100 volts-300 volts. For such high voltages, electricallycontrolled DM voltage sources are often more expensive than theoptically controlled DM voltage source formed from control circuit 34and DC voltage source 36 of FIG. 3.

[0040] An exemplary DC source 36 has a voltage of about 100-300 volts.For such a source a dark-state resistance of about 10¹⁰ ohms ispreferable to avoid substantial power dissipation in the dark-state. Forsuch a resistance, channel region 26 typically has a length of at least0.5 microns and preferably a length of 1-100 microns and a width ofabout 1,000 microns. The channel region 26 is highly inter-digitated toreduce to overall transverse extend of the region 26 (FIG. 4). Suchchannel dimensions also provide breakdown voltages of in excess of 150volts.

[0041] In other embodiments of system 50, photosensitive switch 12 isreplaced by a photosensitive resistor (not shown). The photosensitiveresistor has a photosensitive channel region 26 that includes eitherorganic or inorganic materials. Exemplary inorganic materials includeamorphous selenium (Se), silicon (Si), cadmium sulfide (CdS), andcadmium selenide (CdSe). These inorganic materials may be doped withwell-known electron acceptors or donors.

[0042] Other embodiments of the invention will be apparent to thoseskilled in the art in light of the specification, drawings, and claimsof this application.

What we claim is:
 1. An apparatus, comprising: first and secondelectrodes; a channel having a photosensitive organic material andextending between the electrodes; and a light source positioned toilluminate the channel transverse to a direction of current flow thereinand configured to produce light with a wavelength capable of changingthe conductivity of the material, the channel being configured tooperate as an optically controlled switch.
 2. The apparatus of claim 1,wherein the light source is situated to illuminate the entire length ofthe channel.
 3. The apparatus of claim 2, wherein the channel has aresistance that decreases by at least 10⁴ in response to beingilluminated by the light source.
 4. The apparatus of claim 2, whereinthe channel has a resistance of at least 10⁷ ohms when not illuminated.5. The apparatus of claim 4, wherein the channel has a breakdown voltageof at least 50 volts.
 6. The apparatus of claim 1, wherein the lightsource is a digitally modulated source.
 7. The apparatus of claim 2,wherein the organic material comprises molecules with conjugatedsegments.
 8. The apparatus of claim 7, wherein the material includes oneof an oligomer and a polymer, the oligomer or polymer comprisingphenylenevinylene, fluorene, thiophene, or pyrrole units.
 9. Theapparatus of claim 7, wherein the material includes one of an electronacceptor and an electron donor.
 10. The apparatus of claim 9, whereinthe one of an electron acceptor and an electron donor includes one ofC₆₀, a metal-phthalocyanine, thia-pyrylium, squarylium, an azo-compound,perylene, anthanthrone, and nanocrystalline CdSe.
 11. The apparatus ofclaim 2, wherein the light source is one of an LED and a diode laser.12. The apparatus of claim 1, wherein the first and second electrodesare constructed of the same conducting material.
 13. A system,comprising: a micro-electromechanical (MEM) device; and a circuitconnected to control the MEM device, the circuit including an organicchannel configured to operate as an optically controlled switch.
 14. Thesystem of claim 13, wherein the circuit further comprises: a lightsource positioned to illuminate the channel transverse to a direction ofcurrent flow therein and configured to produce light with a wavelengthcapable of changing the conductivity of the material, the channel beingconfigured to operate as an optically controlled switch.
 15. The systemof claim 14, wherein the light source is situated to illuminate theentire length of the channel.
 16. The system of claim 14, wherein thechannel has a resistance that decreases by at least 10⁴ in response tobeing illuminated by the light source.
 17. The system of claim 14,wherein the channel has a breakdown voltage of at least 50 volts. 18.The system of claim 13, wherein the channel having a doped organicmaterial whose conductivity is responsive to illumination from the lightsource.
 19. The system of claim 18, wherein the organic materialincludes organic molecules with conjugated segments.
 20. The system ofclaim 19, wherein the organic material includes one of an oligomer and apolymer, the oligomer or polymer including phenylenevinylene, fluorene,thiophene, or pyrrole units.
 21. The system of claim 18, wherein theorganic material includes a dopant that is one of an electron acceptorfor the organic material and an electron donor for the organic material.22. The system of claim 21, wherein the dopant includes one of C₆₀, ametalphthalocyanine, thia-pyrylium, squarylium, an azo-compound,perylene, anthanthrone, or nanocrystalline CdSe.
 23. The system of claim14, wherein the MEM device comprises a capacitor; and wherein thecircuit is connected to control a charge state of the capacitor.
 24. Thesystem of claim 23, wherein the MEM device further comprises a reflectorwhose orientation is controlled by the charge state of the capacitor.25. A system, comprising: a micro-electromechanical (MEM) device; and acircuit connected to control the MEM device, the circuit including aninorganic channel configured to operate as an optically controlledphotosensitive resistor.
 26. The system of claim 25, wherein theresistor further comprises: a digitally modulated light sourcepositioned to illuminate the photosensitive inorganic resistor.
 27. Thesystem of claim 25, wherein the MEM device comprises a capacitor; andwherein the circuit is connected to control a charge state of thecapacitor.
 28. A method for producing a drive voltage, comprising:applying a voltage across an organic photosensitive switch; and applyinga light intensity to the organic photosensitive switch while applyingthe voltage, the applied voltage being greater than any photovoltaicvoltage produced by the light intensity.
 29. The method of claim 28,wherein the applying a light intensity comprises modulating the lightintensity to have first and second values during a series of first andsecond periods, respectively.
 30. The method of claim 28, furthercomprising: applying a voltage across a load element, the value of thevoltage being a function of a current in the switch.
 31. The method ofclaim 30, wherein the applying a voltage across a load element producesa voltage across one of a capacitor and an inductor, the one of acapacitor and an inductor being configured to control an orientation ofa MEM device.